The present invention relates to an evaporator system for an industrial boiler, and includes a heat transfer system for generating a water-steam mixture, means for separating water and steam from the water-steam mixture, and means for drying the separated wet steam.
In its most fundamental form such an evaporator system consists of a water-steam drum, a heat transfer section and interconnecting piping. Water from the drum is transported to the heat transfer section where it is partly evaporated. The thus generated water-steam mixture is transported back to the drum, where the steam is separated from the water and the separated steam is dried. Other connections on the evaporator system are for feed water supply and steam extraction.
Conventionally the water-steam drum is a vessel having a relatively large diameter because of the functions it has to fulfill. It is designed to contain the minimum amount of water required among others to guarantee the steam generation of the boiler when the feed water supply to the drum is momentarily interrupted. It is designed to contain the minimum steam volume required among others to have space for a water-steam separator and a steam dryer to realize a guaranteed steam purity at steam extraction and to have space for a water level that shifts to compensate for the fluctuating amount of water contained in the heat transfer section during start-up, shut-down and other load changes of the boiler.
The relatively large diameter in combination with relatively high steam pressures leads to a relatively large wall thickness, which limits the allowable temperature transients related to load changes of the boiler.
From EP-B-0 158 891 (U.S. Pat. No. 4,624,111) a process is known in which downstream of a high pressure steam turbine a water-steam pre-separator, a second water-steam separator and a reheater are connected in series. The saturated steam discharged from the high pressure steam turbine flows through the pre-separator first, then through the second separator, and finally through the reheater. The separated water is led from both separators to a water pre-heater.
It is an object of the present invention to design the separating and drying means of the known evaporator system in such a way that wall thicknesses are reduced and as a result thereof faster load changes of the evaporator system are allowed.